Spinal nucleus implants are known. For example, U.S. Pat. Nos. 5,562,736 and 5,674,295 disclose an implant having a constraining jacket surrounding a hydrogel core. As described therein, a hydrogel material is dehydrated, resulting in an undersized substantially cylindrical gel capsule which is then inserted into the constraining jacket which is then closed to prevent the hydrogel from escaping the confines of the jacket. The implant is rehydrated and conditioned by a series of compressive loads which renders the nucleus body to a partially flattened or oval shape. The implant is then inserted into a retaining tube to maintain the oval shape up until implantation. Alternative embodiments include an outer skin formed by ion implantation which causes outer layer polymerization and functions as the constraining jacket. U.S. Pat. No. 6,022,376 describes an implant made from an amorphous hydrogel polymer core surrounded by a constraining jacket. In one embodiment, the amorphous polymer is poured into one end of the constraining jacket in an unhydrated state, and the jacket then closed. The implant is then massaged to flatten and narrow the implant in preparation for implantation. Alternatively, the amorphous polymer may be injected into the constraining jacket. In one embodiment, an empty constraining jacket is implanted into the disc space and the amorphous polymer is then injected into the constraining jacket. In one embodiment, the amorphous polymer is shaped into a plurality of “microchips” which have been manufactured to have a certain shape. U.S. Pat. No. 6,132,465 is directed to a nucleus implant having a hydrogel core in a constraining jacket. The hydrogel core is inserted into the constraining jacket in a wedge-shaped dehydrated state and then implanted into the nucleus cavity. A final dehydration step is described where the hydrogel core can be forced into certain shapes, i.e., it can be “entirely flat”. U.S. Pat. No. 6,602,291 describes a prosthetic spinal disc nucleus which is made with a hydrogel core having a first shape in the hydrated state. It is then placed in a constraining jacket and reshaped to have a second shape in the dehydrated state. The core is configured to transition from the second shape to the first shape on hydration. The second shape may include an elongated shape defined by a leading end, the hydrogel core tapering from the central portion to the leading end, to facilitate insertion through an opening in the annulus. An inherent shape memory attribute is said to be obtained by pouring a hydrogel material, suspended in a solvent into a mold having a shape corresponding to the desired hydrated shape. After a solvent exchange process, the hydrogel core is dehydrated in an oven and inserted into a constraining jacket. The implant is then rehydrated and subjected to conditioning steps by exposure to at least three compressive loads. The implant is then reshaped and dehydrated, i.e., it is placed into a mold having a streamlined shape and then placed in an oven to expedite dehydration of the hydrogel core, which causes the implant to have a streamlined shape. The implant may be compressed while dehydrating, The implant is then maintained in the dehydrated shape prior to implantation. U.S. Pat. No. 6,533,817 is directed to a packaged, partially hydrated prosthetic disc nucleus which includes a prosthetic disc nucleus and a retainer. Upon contact with a hydration liquid, the retainer is said to be configured to allow the hydrogel core to hydrate from the dehydrated state but prevents the core from hydrating to the final hydrated state, i.e., the prosthetic disc nucleus is constrained by the retainer to a partially hydrated state. As described therein, a hydrogel core is formed and placed within a constraining jacket. The prosthetic disc nucleus is then dehydrated, preferably under compression within a compression mold and the entire assembly is placed in an oven. As the core dehydrates the compression mold forces the nucleus to a desired dehydrated shape in the dehydrated state. The dehydrated disc nucleus, in the dehydrated state is then placed in the retainer. The packaged disc nucleus can then be exposed to a hydration liquid where it transitions to the partially hydrated state. Once removed from the retainer, the disc nucleus, in the partially hydrated state is implanted into the disc space. U.S. Pat. No. 5,047,055 is directed to a hydrogel intervertebral disc nucleus. As described therein, a prosthetic nucleus for a disc is composed of a hydrogel material. The nucleus is made by mixing polyvinyl alcohol with a solvent heating the mixture and then poured or injected into a mold. The shaped hydrogel can be dehydrated for implantation. Other hydrogel materials are also described which can be shaped by cast molding or lathe cutting. The volume of the nucleus is said to reduce by about 80% when dehydrated and that the rigidity of the dehydrated nucleus will help the surgeons to manipulate the nucleus during an operation. U.S. Pat. No. 5,534,028 is directed to a hydrogel intervertebral disc nucleus with diminished lateral bulging and describes certain hydrogel treatment procedures which are similar to those disclosed in U.S. Pat. No. 5,047,055, e.g., see the implantation discussion at column 11, lines 25-40.
A potential shortcoming of artificial disc replacements is the propensity for extrusion of the implant through the annulus. The nucleus pulposus is held in place by the annulus in vivo. However, the annulus must be compromised in order to gain access to the diseased or damaged disc space. The resulting annular defect provides a path of least resistance through which an implant may travel under extremes of load and/or motion.
The likelihood of extrusion occurring may be increased by a poor implant cross-section to annular incision size ratio. The higher this ratio, the less likely it is that the implant will extrude. For example, if a 5 mm ø implant is placed into the disc space through a 5 mm ø incision the implant cross-section to annular incision ratio is 1.0 and extrusion is highly likely. It is therefore advantageous to keep this ratio as high as possible by reducing the incision size. This can be facilitated by decreasing the cross section of the implant which must pass through the annulus. In designing implants to be used with minimally invasive techniques, the cross-sectional area of the implant should be as small as possible. Although some of the above-described implants are dehydrated and shaped in some manner, none of them are dehydrated and reshaped so as to force the implant to assume an implantation-friendly shape substantially different from the final, hydrated implanted shape. Thus, the implant's original footprint may be maintained in the form of a wafer, which may have an aspect which is decreased along one axis, but not the other. Alternatively, isotropic shrinkage from dehydration may be effected which does not alter the topography of the implant. In the case of simple dehydration, the cross-sectional area is equal to the hydrated cross-sectional area divided by the expansion ratio.
In addition, production of dehydrated wafers requires unyielding mold fixtures made, e.g., from stainless steel in order to equalize the load while constraining the implant to a particular dimension. Water vapor must be transported from the implant in the wafer producing fixtures in order to achieve dehydration. Since metal is typically not porous, this results in long drying times, since the vapor transport path is long.
Another method of optimizing the implant cross section for minimally invasive surgery is partial hydration of a hydrogel material which allows for manipulation of the implant by the surgeon with or without specialized tools designed for this purpose. There are a number of potential drawbacks to partial hydration or plastification such as incompatibility of the plasticizer used with the sterilization method, difficulty of retaining the required amount of plasticizer within the package over extended periods and the possibility of creep occurring during storage.
The present invention addresses at least these problems by providing a spinal nucleus implant of novel configuration which has been dehydrated and reshaped in multiplanar directions utilizing a novel compression system.